The invention relates generally to the field of power over local area networks, particularly Ethernet based networks, and more particularly to a method of measuring cable impedance via the use of power over Ethernet.
The growth of local and wide area networks based on Ethernet technology has been an important driver for cabling offices and homes with structured cabling systems having multiple twisted wire pairs. The structured cable is also known herein as communication cabling and typically comprises four twisted wire pairs. In certain networks only two twisted wire pairs are used for communication, with the other set of two twisted wire pairs being known as spare pairs. In other networks all four twisted wire pairs are used for communication. The ubiquitous local area network, and the equipment which operates thereon, has led to a situation where there is often a need to attach a network operated device for which power is to be advantageously supplied by the network over the network wiring. Supplying power over the network wiring has many advantages including, but not limited to; reduced cost of installation; centralized power and power back-up; and centralized security and management.
Several patents addressed to the issue of supplying power to a PD over an Ethernet based network exist including: U.S. Pat. No. 6,473,608 issued Oct. 29, 2002 to Lehr et al.; and U.S. Pat. No. 6,643,566 issued Nov. 3, 2003 to Lehr et al.; the contents of all of which are incorporated herein by reference.
The IEEE 802.3af-2003 standard, whose contents are incorporated herein by reference, is addressed to powering remote devices over an Ethernet based network. The above standard is limited to a powered device (PD) having a maximum power requirement during operation of 12.95 watts. Power can be delivered to the PD either directly from the switch/hub known as an endpoint power sourcing equipment (PSE) or alternatively via a midspan PSE. In either case power is delivered over a set of two twisted pairs. The above mentioned standard further prescribes a method of classification having a total of 5 power levels of which classes 0, 3 and 4 result in a maximum power level of 15.4 Watts at the PSE which is equivalent, in the worst case, to the aforementioned 12.95 watt limit.
The actual difference between the power level drawn from the PSE and the power level received at the PD is primarily a function of the power lost in the cable. The power required at the PSE to support a particular requested maximum power at the PD is thus equal to the requested maximum PD power plus any losses due to the impedance between the PSE and the PD. A maximum cable length of 100 meters is specified, and the voltage supplied by the PSE may range from a minimum of 44 volts to a maximum of 57 volts as measured at the PSE output. Thus, the amount of power lost in the cable may vary significantly depending on actual cable length and actual voltage.
The total amount of power available in a system supporting a plurality of PDs is often limited to less than 15.4 watts times the number of PDs attached. Thus, it is important to manage the power allocated to each PD so that the total power drawn does not exceed the power available from the power supply. In the event power is allocated according to classification, a measure of the impedance between the PD and the PSE, which is a metric of the power loss in the cable, would thus give a more accurate allocation of power per PSE port, since the power lost in each cable would be determined and not reserved for a worst case scenario. Thus, in the event of a determined impedance lower than the worst case scenario, a PD exhibiting a maximum power requirement of 12.95 watts may be supported with an allocation of less than 15.4 watts from a common power supply.
A PSE according to the above mentioned specification exhibits certain characteristics including a maximum current during startup mode. A PD according to the above mentioned specification exhibits certain characteristics including a detection resistance, optional classification current, polarity independence, minimum input capacitance, and a minimum turn on voltage. In the event of a PD input capacitance being greater than or equal to 180 μF, the PD is further required to control the inrush current during startup mode so as to ensure a maximum Iinrush of 400 mA. After detection and optional classification, the PD is not to close an isolating switch placing the input capacitance and load across the PSE until a minimum turn on voltage is detected.
There is thus a long felt need for a method of determining the impedance between the PSE and the PD, the impedance being primarily a function of cable quality and cable length.